1. Field of the Invention
The present invention relates to a ceramic powder for a green sheet and a multilayer ceramic substrate. Specifically, the present invention relates to a ceramic powder for a green sheet that gives a low-temperature fired ceramic substrate with excellent properties in higher frequency bands such as microwave and millimeter-wave bands in the fields of information communications, automobiles, etc., and to a multilayer ceramic substrate.
2. Description of the Related Art
Low-temperature fired multilayer ceramic substrates are in practical use as a technique for supporting high-level information communications. Low-temperature fired multilayer ceramic substrates are obtained by forming a circuit pattern with a conductor paste on a green sheet prepared from a ceramic powder for a green sheet, laminating and integrating a plurality of the green sheets into one, and firing the laminate, which is a multilayer substrate that contains circuit wiring incorporated therein. Low-temperature fired multilayer ceramic substrates are also called co-firable substrates since the green sheet (ceramic material) and the conductor paste (conductor material) are fired simultaneously.
The ceramic powder for a green sheet to be used in preparation of the low-temperature fired multilayer ceramic substrate is generally a mixture of a glass component and a crystalline inorganic compound such as alumina. Here, as low-temperature firing at about 1,000° C. or lower brings about rapid densification as glass components soften in the sintering process, it is possible to select glass components with softening points of 1,000° C. or lower.
In recent years, in the information communications field, the radio wave frequency bands used have become higher with the augmentation of communications equipment and the increase in the number of channels, and microwave and millimeter-wave bands are now being used. The higher the frequency of the radio waves used, the more the radio waves transform into heat in the circuit, that is, the higher the transmission loss. Therefore, users who desire higher performance of a product require a decrease in transmission loss in the higher frequency bands. Factors that greatly influence transmission loss in the higher frequency bands include the dielectric properties of the ceramic substrate and the electric conductivity of the conductor. Of those, the dielectric properties of the ceramic substrate are particularly important, and the contribution ratio is higher, the higher the frequency. Therefore, in order to reduce transmission loss in the higher frequency bands, a ceramic powder is required for a green sheet that gives a ceramic substrate with excellent dielectric properties (that is, with low dielectric constant εr and dielectric loss tangent tanδ).
However, it is generally difficult to balance the dielectric properties of a ceramic substrate with low-temperature firing. In particular, in the case of using Ag, with excellent electric conductivity, as a conductor material, as low-temperature firing at 900° C. or lower is required, it is even more difficult to balance the dielectric properties with the low-temperature firing.
Examples of a conventional ceramic powder for a green sheet include a ceramic powder for a green sheet consisting of 12 to 59.6% by weight of alumina, 18 to 69.6% by weight of borosilicate glass, 1 to 40% by weight of anorthite crystal, and 1 to 5% by weight of celsian crystal (see JP 6-305770 A, for example). Such ceramic powder for a green sheet can be fired at a low temperature regardless of firing conditions and can give a ceramic substrate with a low dielectric constant εr and excellent mechanical strength.
In addition, several reports have disclosed the compositions of aluminosilicate glasses with low dielectric constant εr and dielectric loss tangent tanδ (see JP 11-292567 A and “Glass fiber and glass cloth” by Keita Miyazato et al., Journal of Japan Institute of Electronics Packaging, Vol. 4, No. 2, P. 98-101, for example).
However, JP 6-305770 A does not mention the problems of transmission loss in the higher frequency bands (such as microwave and millimeter-wave bands) and a method for achieving a good balance between dielectric properties and low-temperature firing. In fact, a ceramic substrate produced from the ceramic powder for a green sheet of JP 6-305770 A has poor performance as a substrate for the higher frequency bands because of a larger transmission loss in the higher frequency bands compared with the conventional material, i.e., an alumina substrate. In general, the transmission loss of alumina in the higher frequency bands is small compared with that of a borosilicate or aluminosilicate glass, and the transmission loss in the higher frequency bands can be qualitatively reduced to a certain degree by increasing the amount of alumina incorporated into the ceramic powder for a green sheet of JP 6-305770 A. However, as such a ceramic powder for a green sheet may cause problems in terms of water absorption properties and lack of strength, a glass component with a small transmission loss in the higher frequency bands is desired.
On the other hand, JP 11-292567 A and “Glass fiber and glass cloth” by Keita Miyazato et al., Journal of Japan Institute of Electronics Packaging, Vol. 4, No. 2, P. 98-101 disclose the compositions of aluminosilicate glasses with low dielectric constant εr and dielectric loss tangent tanδ. However, glass compositions with the compositions are intended to be used for glass fibers and have high softening temperatures and high viscosities even after softening, and the compositions are inappropriate as components of the ceramic powder for a green sheet.
At the beginning, the inventors of the present invention prepared a ceramic powder for a green sheet containing the glass composition of JP 11-292567 A and alumina powder at various mix ratios and evaluated low-temperature fired ceramic substrates produced from the ceramic powder for a green sheet on their porosities and dielectric properties in microwave bands. Meanwhile, a low-temperature fired ceramic substrate was prepared from the ceramic powder for a green sheet of JP 6-305770 A as a control sample and evaluated in the same way as above. As a result, the low-temperature fired ceramic substrate produced from the ceramic powder for a green sheet including the glass powder of JP 11-292567 A was found to have good dielectric properties and in most cases, small transmission loss compared with the low-temperature fired ceramic substrate of JP 6-305770 A.
However, in the case where the ratio of glass components incorporated is in the range of 40 to 70 vol %, the porosity of a general low-temperature fired ceramic substrate such as the substrate of JP 6-305770 A is about 2%, while the porosity of a low-temperature fired ceramic substrate prepared from the glass powder of JP 11-292567 A is as high as about 20%. That is, a low-temperature fired ceramic substrate prepared from a ceramic powder for a green sheet containing the glass powder of JP 11-292567 A is considered to have high hygroscopicity and does not have the environmental resistance required of a product. The increase in the porosity of a low-temperature fired ceramic substrate is assumed to be caused by high viscosity after softening in the case of the glass composition of JP 11-292567 A. That is, in order to prepare a dense ceramic, a process for allowing a softened glass component to flow into the spaces in the alumina powder is required, but in the case where the viscosity of the glass component is high, it is probably impossible to complete the process within a practical time frame.
Meanwhile, several ceramic powders for a green sheet, which were studied while varying the glass composition, had problems of warping and creasing due to changes in shrinkage behaviors caused by lowering of the softening point of a glass component in the vicinity of the conductor because of the dispersion of silver in the steps of laminating and integrating a plurality of green sheets with circuit patterns formed therein with a silver-based paste and performing co-firing. The warping and creasing can be suppressed by incorporating an alkali metal oxide (such as Na2O or K2O) into the glass components. However, as described in “Glass fiber and glass cloth” by Keita Miyazato et al., Journal of Japan Institute of Electronics Packaging, Vol. 4, No. 2, P. 98-101, incorporation of an alkali metal oxide increases transmission loss in the higher frequency bands (that is, it decreases dielectric properties).